1. Field of the Invention
The present invention relates to a method for preventing a compressor from freezing in an automobile fuel cell system in which water contained in a gas discharged from a fuel cell is used for humidifying process air.
2. Description of the Related Art
A fuel cell system in which water is supplied to humidify the process gas is known and disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 7-14599. Such a fuel cell system is shown in FIG. 2 in the attached drawings in which air is sucked from an air feeding conduit 30 and supplied to a fuel cell 33 after it has been pressurized to a certain pressure by a compressor 32 driven by a motor 31. Oxygen in the air is consumed in the fuel cell 33 and the gas is released to ambient air as discharge gas after being expanded by an expansion device 35 coupled to the compressor 32 and the motor 31 by a common shaft 34.
On the other hand, water produced and contained in the discharge gas is collected into an open type reservoir 39 after being separated from the discharge gas by liquid separators 37 and 38 provided in an air discharge conduit 36, and is delivered to a jet nozzle 41 by a pump 40 to be injected into the air feeding conduit 30 for humidifying the process air.
That is, in the above-mentioned fuel cell system, the produced water contained in the discharge gas from the fuel cell 33 is separated and collected, and used for humidifying the process air.
When the compressor 32 is operated to enable the fuel cell 33 to supply electric power, the collected water circulates while being maintained at a somewhat higher temperature due to the heat of reaction. If the fuel cell system is stopped, all the water circulating through the fuel cell 33 is not collected but is liable to remain especially in the interior of the compressor 32 or the water feeding path.
Accordingly, when the fuel cell system is used in a cold district, the remaining water in such a state may freeze to block the water path or break the compressor 32 when the system is restarted. Of course, to avoid such a trouble, a countermeasure may be considered in that the operation of the compressor 32 continues for a predetermined period of time after the supply of water has interrupted so that the remaining water is discharged as much as possible. In the fuel cell system for an automobile, however, such a period of time necessary for stopping the compressor becomes considerably long because it must includes a large margin to cover the variation of rotational speed of the compressor due to the change of an engine output and/or a stored amount of electric power in a secondary battery, resulting in the increase in energy loss.
The object of the present invention is to solve the above described problems and to provide a method for assuredly preventing the compressor from freezing, in a simple manner, in the fuel cell system.
To solve the above-mentioned problem, a method for preventing a compressor from freezing in an automobile fuel cell system is provided, which system comprises a fuel cell having an air feeding tube and an air discharging tube, the compressor connected to the air feeding tube, a regenerator connected to the air discharging tube, a water separation tank disposed in the air discharging tube before the regenerator, and a water supply passage to supply water collected in the water separation tank to the compressor. According to the first aspect of the present invention, the method comprises the steps of: issuing a command to block the water supply passage when the output of the fuel cell is to be stopped while the compressor is operating; detecting humidity in a process air discharged from the compressor after the water-feeding path is blocked; and issuing a command to stop the compressor when the detected humidity decreases to a preset value.
Water supplied to the compressor is evaporated by the compressed and heated air, and as a result the process air is humidified by the vapor thus obtained. If the water supply path is blocked prior to the stop of the compressor, the humidity in the process air discharged from the compressor promptly decreases, whereby the detected humidity soon lowers to a preset value and the command for stopping the compressor issues at this time. Since the detection of such a low humidity (the preset value) means that the moisture in the interior of the compressor including the water-feeding path has been discharged as much as possible, it is possible to assuredly prevent any accident from occurring due to the freezing of the remaining water, without useless energy loss.
According to the second aspect of the present invention, the presence of the remaining moisture in the compressor is detected by the detection of the temperature of the process air. If the water supply is interrupted, the humidity to be added to the process air also decreases, which simultaneously lowers the capacity for cooling the process air. Thus, it is similarly possible to prevent any accident from occurring due to the freezing of the remaining water, by using the preset temperature value.